Is flat frequency response a hoax?

I have seen a number of speaker companies claiming that their design has a ruler flat response. They then proudly display a graph with a flat line as if it's an indication of its superior quality. Poppycock.

Flat response is a myth. Everybody has different hearing so there's no reason to expect that there would be a one size fits all frequency response.
We don't all wear the same sized shoes do we? Everybody has different sized feet. 

There are plenty of flat response speakers that sound horrible. I've heard quite a lot. There are many non ruler flat great sounding speakers too.
Do not be duped by this hogwash. 

If the mixing engineer used a pair of speakers that is 3db down in the mids and top end then he may have boosted that region by a similar amount. Now when the track gets played on a ruler flat speaker it sounds too bright. So as you can see, a flat response is useless.

Your hypothetical mastering engineer, who masters on a single flawed speaker and, what, hasn’t even bothered to zero out the frequency response before analysis, is ludicrous. This hypothetical person/situation is, as you say, “poppycock”.

A circular, self-referential straw man argument—precisely like the one kenjit set up with this caricature of a completely unprofessional mastering engineer—is a worthless rhetorical device. If your goal is to arrive at truth, this is a disingenuous way of going about it.

In reality, a professional mastering engineer reviews program material across numerous grades and styles of speakers; from mobile devices to planar magnetic/electrostatic, to massive vintage JBLs or Altecs, and even gigantic audiophile multiway towers like Dynaudios and Wilsons.

The situation kenjit posits, a poorly mastered recording made by an unprofessional engineer, is useless because such a poor recording will only sound correct on a system with the same flaws as the original one that the recording was mastered on.

This is precisely why a professional mastering engineer first zeros a mastering system to a reference standard before going to work. It is also why this entire thread is pointless.
A flat frequency response is exactly what you want if you want to avoid any obvious character in a loudspeaker. Once you become aware of sonic anomalies in a loudspeaker it's difficult to forget them and let that speaker disappear.

The fact that many favourite recordings of yours may have been recorded using loudspeakers with a non ruler flat FR is unfortunate. They won't, and they can't sound the same with different loudspeakers. Unless you know exactly where the anomalies occur and can correct them with DSP, but it's probably best to not go there - use tone controls instead.

Many recordings back in the 60s and 70s were done using JBLs in the US and large Tannoys in the UK and perhaps this explains the popularity of these loudspeakers to this day. 

It begs the question why there isn't a BBC type frequency response standard for the use of monitoring loudspeakers in recording studios. Ditto for microphones.

The fact your room may also affect FR is something else you also need to be aware of. 

To say that a flat frequency response matters most is a different argument entirely. Sometimes heavy damping is used to flatten the frequency response but unfortunately it tends to also flatten the dynamics and the life out of the sound too.

As then there's the question of 'how flat is ruler flat?'  Perfect pitch anybody?

Following millercarbon’s comments on loudness compensation, Yamaha has made “variable loudness” controls on their receivers and amps, mostly mid-fi admittedly, for decades. Few if any high end companies have tried to solve this particular problem, even McIntosh. I’d like to see something like this: a contour control where the input sensitivity and output gain can be equalized to calibrate the tonal correction properly. Otherwise you get too much or to little boost on each input source and recorded level, and no way to adjust for speakers’ sensitivity room size etc. Yamaha has come closest to this,
but it could be done even better. 
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People are free to waste their time as they please but good luck convincing the dogmatic of anything they choose to not believe, including conspiracy theories about flat frequency response.

Hey look people lie about most anything. Why should frequency response be any different?

Now back to listening to my gear,which by happens to have very flat frequency response on the grand scale of things, with my golden ears.....